Process for preparing zirconium oxides



'July 6, 1965 `l. c. KLIMASZEWSKI 3,93,346

PROCS-S FOR PREPARING ZIRCONIUM OXIDES 'Filed March e, 1962 To STACK 210(N03)2 HN03 H20 scau Towee.

' f evAPonAToR Noz PUMP 2 HNO;

vdv J\ vols-INES P 3g HNO; v H20 VENT G'As STREAM z-roz FINES AIR ITOZ PUM f-FLTRATE IN VEN TOR. /IQV//Y C. Kl/MASZEWSK/ United States Patent O 3,193,346 PiGCil-SS FR PREPARENG ZHRCONEUM @REDES livin C. Klimaszewski, Pensacola, Fla., assigner, by mesue assignments, to Pittsburgh Plate Glass ornpany File-d Mar. 6, 1962, Ser. No. H1896 3 Claims. (Qi. 23-14il) This application is a contlnuation-in-part of my coending application, Serial No. 819,093, tiled lune 9, 1959, now abandoned.

This invention relates to the production of chemicals and more particularly to the production of zirconium dioxide from a zirconyl nitrate solution. The principal obiect of the present invention is to provide an improved process for the production of zirconium dioxide with high etliciency and excellent recovery of lay-product nitric acid.

StiH another object of the invention is to provide a process of the above type wherein maximum productivity can be obtained from the process equipment with a minimum loss of zirconium values.

(Ether objects of the invention will in part be obvious and will in part appear hereinafter.'

The invention accordingly comprises the process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing which is a schematic llow sheet.

The process of the present invention will be particularly described in connection with its use as a part of an overall process for the production of high purity zirconium particularly adapted to nuclear reactor use. ln the purification of zirconium for reactor use, it is essential that sub stantially all hafnium be removed therefrom. One process for achieving such separation embodies a solvent extraction of Zircon-yl nitrate with an organic solvent such as tributyl phosphate. In such a process, the purified zirconyl nitrate is then removed from the organic phase by Washing with an aqueous acid solution.

The resultant aqueous acid solution is then concentrated to precipitate zirconyl nitrate crystals. The slurry of crystals is transferred to a filter feed tank from which it is fed to a filter Where the zirconyl nitrate crystals are separated from the mother liquor. The mother liquor is recycled to tie evaporator and the filter cake containing the basic zirconium nitrate or nitrate of higher zirconitun content (zirconyl nitrate) crystals is fed to a caicining kiln wherein the zirconyl nitrate is decomposed to zirconium dioxide and oxides of nitrogen.

The calcining operation inevitably generates some fine zirconium dioxide dust which leaves the kiln in the vent stream of air and oxides of nitrogen. Since these zirconium dioxide lines contain valuable highly puriiied zirconium, they are recovered by scrubbing in a jet condenser. The thus recovered fines are fed back to the lter feed tank containing the zirconyl nitrate slurry.

In a preferred embodiment oft re invention, the concentration o the zirconyl nitrate solution is achieved in an evaporator having an associated steam-heated reboiler. Since the above described process also involves a high degrec of recycle between the evaporator tank and the effluent from the jet condenser, it is desired that a small bleed stream be sent forward into the kiln to prevent the undue accumulation of impurities in the iiltration recycle system.

Referring now to the drawing, there is illustrated one preferred embodiment of process equipment for practicing the present invention. As illustrated, the aqueous solution of nitric acid and zirconyl nitrate is introduced into flggdb Patented July 6, 1965 an evaporator l0 wherein the solution is concentrated by boidng off vapors of nitric acid and water. These vapors are condensed in a condenser l1 andthe resultant liquid is then fed to a storage tank l. The solution in the evaporator l0 is continuously recycled by means of a large pump /i through a reboiler i2 Where an elevated temperature is maintained by means of the steam coil. The zirconyl nitrate crystals which collect on the bottom of the evaporator 10 are transferred from the evaporator 10 to a lilter feed tank iS by means of a suitable transfer pump 25.6. From the feed tank ld the zirconyl nitrate crystal slurry is transferred by means of a pump Ztl through a cooler 22 to a drum lilter 24. The basic nitrates of zirconium (zirconyl nitrate or nitrates of a higher zirconium content than zirconyl nitrate) as a filter cake is fed from the filter 24 to a kiln 26 Where it is heated to a sufficiently elevated temperature to decompose the nitrate to zirconium dioxn ide and oxides of nitrogen. Air is preferably passed through the kiln so that the vent gases leaving the kiln contain Water, nitric acid, oxygen, oxides of nitrogen and nitrogen. T -ese gases are quenched in a jet condenser 2d wherein some of the oxides of nitrogen are converted to nitric acid and all of the zirconium dioxide iines in the gas stream from the kiln are scrubbed out of the gas stream and are collected in a storage tank 30 where the eilluent liquid from the jet condenser is separated from the residual gases. These residual gases then pass through a scrub tower 32 Where most of the oxides of nitrogen are recovered as nitric acid. The jet condenser effluent storage tank 30 will contain nitric acid, Water and some valuable zirconium dioxide lines. Accordingly, a portion of this liquid is recycled, by means of pump 34, through a cooler 36 back to the filter feed tank 1S. Another portion or" the liquid is fed to the iet condenser 2d to serve as the Working liquid in the jet condenser.

In a preferred embodiment, `a portion of the mother liquor from the filter 24 is pumped by means of pump 4l) in a small bleed stream to the calcining kiln 26. The major portion of the mother liquor is returned to the evaporator 1 0.

As indicated iu the drawing, the mixture of nitric acid and Water, which is distilled from the zirconyl nitrate solution in the evaporator tank, is condensed and stored in the storage tank 13. In a preferred embodiment of the invention this liquid is used as a scrub liquid along with some additional water in the scrub tower 32 to remove the residual oxides of nitrogen from the kiln vent gases.

It should be apparent, from the above consideration of the process, lthat there is excellent economy of zirconium values and substantially complete recovery of the nitrogen content in the zircouyl nitrate solution feed to the evaporator lil. Thus, the recovered nitric acid can be utilized in a preceding step in the process for the formation of the zirconyl nitrate feed for the extraction systemi Example I ln one preferred method of operating the abovedescribed apparatus, the feed stream to the evaporator contains about .2S-.55 pounds zirconium/gal. and about lit-24% HNO3. The evaporator is run at about 240- 255 F., so that the vapor stream going to condenser lll contains about lll-25% HNO3.

The feed to the evaporator is 6 gpm. and there is about an 8-hour residence time for the liquid in the evaporator. About 3 gpm. leaves the evaporator as vapor and about 3 gpm. `leaves as concentrated slurry. In one preferred embodiment, the circulating pump 14 between the evaporator lil .and reboiler 12 is a very high capacity pump of about 1100 gpm. With the 3 gpm. feed of slurry to the lter feed tank, about 0.3 gpm. of

eight (8) hours mentioned. If a dense zirconium oxide product is desired, thetresidence can be increased to 10 to 30 hours or more, as described in my copending ap' plication, Serial No. 819,839, tiled June' '12, 1959.

Equally, the temperature and the pressure of the slurry in the evaporator can be varied substantially, as can the flow rates to and from the evaporator. Similarly, the method of heating tlie slurry, the temperature ofthe calcination step and the feed i'ates to the filter are not critical. In fact, centrifugal separation of the crystals can be employed in lieu of filtration.

Although the present invention has been described With reference to the specific details of certain embodil ments, it is not intended that such details :shall be regarded `as limitations upon the scope of the invention except insofar as included in the accompanying claims.

I claim:

1. In the process of preparing zirconium oxidev from a solution of zirconyl nitrate and nitric acid wherein the solution is concentrated in an evaporator to kprecipitate some .solid zirconyl nitrate with subsequent filtration of the solid zirconyl nitrate and calcination of the solid zirconyl nitrate to zirconium oxide the improvement which comprises recovery finely-powdered zirconium oxide resulting from the calcining step and recycling said finelypowdered zirconium oxide vso recovered to the filtration step.-

:2. The process of preparing zirconium` oxide from a solution of i Ezirconyl nitrate and nitric aci'dwhich comprises'the steps of concentrating the solution in'anev'apo rator to precipitate some of the zirconyl nitrate, transferring a portion of the resultant slurry to a separation step to remove solid zirconyl nitrate and form a mother liquor, recycling the mother liquor from the separation step to the evaporator, calcining the solid zirconyl nitrate so removed to zirconium oxide, recycling to the separation step finely-powdered zirconium recovered from the calcining step and feeding'a portion of the mother liquor recycle stream formed in the separation step to the calcining step.

3. The proces-s of preparing zirconium oxide from a solution of zirconyl' nitrate in' nitric acid which comprises the steps of maintaining a body of the solution in an evaporator at an elevated temperature to boil oi nit-ric acid and Water to concentrate the solution and thereby to precipitate some' of the zirconyl nitrate, withdrawing a slurryV of solid"zirconyl nitrate from the evaporator, separating the solid zirconyl nitrate, calcining the solid zirconyl'nitrate to zirconium oxide, recovering and recycling nely-powdered zirconium oxide produced in the calcining step to the lseparation step, and feeding fresh solution of zirconyl nitrate to the evaporator at a rate sulcient to compensate for vapors and slurry removed from the evaporator.

References Cited by the Examiner UNITED STATES PATENTS 2,285,443 I6/42 Kinzie et al 23-24.l 2,579,107 12/'51 Bertolus 23-17 3,090,670 '15/ 63 Stambough et al 23-140 OTHER REFERENCES Blumenthal: The Chemical Behavior of Zirconium, D. VanNostrand Co., Inc., New York, 1958, pages 285- 286.

Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 7, Longmans, Green & Co., New York, l1927, pages 124, l6l, 162.

MAURICE A. BRINDISI, Primary Examiner. 

1. IN THE PROCESS OF PREPARING ZIRCONIUM OXIDE FROM A SOLUTION OF ZIRCONYL NITRATE AND NITRIC ACID WHEREIN THE SOLUTION IS CONCENTRATED IN AN EVAPORATOR TO PRECIPITATE SOME SOLID ZIRCONYL NITRATE AND CALCINATION OF THE SOLID ZIRCONYL NITRATE TO ZIRCONIUM OXIDE THE IMPROVEMENT WHICH COMPRISES RECOVERY FINELY-POWDERED ZIRCONIUM OXIDE RESULTING FROM THE CALCINING STEP AND RECYCLING SAID FINELYPOWDERED ZIRCONIUM OXIDE SO RECOVERED TO THE FILTRATION STEP. 